1. Field of the Invention
This invention relates to a printed wiring board incorporating an electronic component which radiates heat by itself, particularly to a semiconductor device having a heat radiating plate, and more particularly to a printed wiring board having a heat radiating means capable of effectively radiating heat from the semiconductor device.
2. Description of the Related Art
Conventionally, for example, a printed wiring board formed as a hybrid IC used in an AC/DC or DC/DC converter circuit incorporates a resin-packaged semiconductor device which includes a semiconductor chip generating more heat during the operation and is mounted on a circuit board. Such a semiconductor device includes a heat radiating plate which radiates externally heat generated by the semiconductor chip.
An example of the printed wiring board incorporating a semiconductor device having the heat radiating mounted on a circuit board is shown in FIG. 7. FIG. 8 is a sectional view taken along line VII—VII in FIG. 7. In a hybrid IC 100 illustrated in FIG. 7, as shown in FIG. 8, a resin-packaged semiconductor device (hereinafter simply referred to as a semiconductor device) 1 is mounted by its surface on a circuit board 102. The semiconductor device 1 incorporates a metallic heat radiating plate 11 which is partially exposed externally from a resin package 10.
The semiconductor device 1, as seen from FIG. 8, includes a semiconductor chip 12, a plurality of inner leads 13 (only one is shown in FIG. 8) which are electrically communicated with the semiconductor chip 12, a resin package 10 which wraps the semiconductor chip 12 and the inner leads 13, a plurality of outer leads 14 (only one is shown in FIG. 8) which are successive to the inner leads 13 and extended out from the resin package 10, and a heat radiating plate 11 which is built in the resin package 10 so as to extend out from the resin package 10 so that heat generated from the semiconductor chip 12 is externally radiated.
On the other hand, as shown in FIG. 7 on the circuit board 102, a wiring pattern (not shown) corresponding to each of the outer leads 14 is formed, and a copper pattern 20 is formed on which the heat radiating plate 11 of the semiconductor device 1 is formed.
As seen from FIG. 8, an upper electrode 12a is formed on the upper surface of the semiconductor chip 12. This upper electrode 12a is electrically connected to the upper face of each of the inner leads 13 through a wire 15. The inner lead 13 is extended out from the resin package 10 and bent to constitute the outer lead 14. The outer lead 14 is soldered to the wiring pattern.
As seen from FIGS. 7 and 8, The above heat radiating plate 11 includes a square die pad 11a and a heat radiating portion 11b for radiating heat generated when the semiconductor device 1 mounted in the circuit board 102 is driven. The semiconductor chip 12 is bonded on the die pad 11a. The lower surfaces of the die pad 11a and the heat radiating portion 11b are soldered on the copper foil pattern 20.
However, since the copper foil pattern 20 and the circuit board 102 have a smaller specific heat than air, the heat from the heat radiating plate 11 is apt to conduct to the circuit board 102 through the copper foil pattern 20 and is stored there. In order to prevent storage of heat in the circuit board 102 to radiate heat effectively, it is possible to propose to increase the area of the copper foil pattern 20. However, an increase in the area of the copper foil pattern 20 leads to an increase in the area of the circuit board 102. In order to satisfy the demand of reduction in the size and thickness of each of various appliances, the reduction in the size and thickness has been demanded also in an electronic component and a printed wiring board. However, the increase in the area of the copper foil pattern 20 can not satisfy the demand described above.